Timing accuracy is important in various network applications, such as wireless base station synchronization, etc. Packet networks operate over optical networks which are evolving to use Optical Transport Network (OTN), Flexible Optical (FlexO), and coherent optics. Frequency synchronization across a network was been previously performed with Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH), and synchronous Ethernet. Timing distribution has been evolved to use IEEE 1588 to enable phase and time synchronization across a network. Timing accuracy is more difficult with OTN and coherent optics with OTN having asynchronous mappings and coherent optics introducing variable delays which may be different in a transmit and receive direction.
Timing synchronization between nodes in a network is described in various standards such as IEEE 1588-2008 “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems,” ITU-T G.8265.1/Y.1365.1 (07/14) “Precision time protocol telecom profile for frequency synchronization,” ITU-T G.8275.1 “Time and Phase Profile,” the contents of each is incorporated by reference herein. The requisite information for the transfer of precise time is (1) a time reference point, or “significant instant” to which timing information can be related, (2) the timing information itself, and (3) a measure of the delay it takes to transfer the timing information between two nodes. The transfer of time over optical networks can be challenging because of delays introduced by elastic First-In-First-Out (FIFO) or queues required in transport mapping schemes such as OTN and enhanced high coding gain Soft-Decision Forward Error Correction (SD-FEC) schemes. Variability in these delays (at start-up, or after fault recovery) can result in different delays in the forward and reverse direction, referred to as delay asymmetry. When transferring time across a network, the delay asymmetry results in a time error.
IEEE 1588-2008 is referred to as Precision Time Protocol (PTP) and is used to synchronize clocks throughout the network. IEEE 1588-2008 defines a protocol for transferring time information over a packet network. It does not address performance aspects such as the time accuracy that can be achieved over a network. Based on current 100 Gb/s system designs, measurements have shown that the optical transport equipment can contribute hundreds of nanoseconds of time uncertainty over a single network hop. Measurements of some off-the-shelf components have shown much worse performance. Newly developed standards have not yet addressed time uncertainty and jitter that can be introduced by SD-FEC type of schemes added on the line side in optical modem/Digital Signal Processing (DSP) devices. These schemes and devices have non-deterministic timing therein.
Commonly-assigned U.S. Pat. No. 9,432,144, issued Aug. 30, 2016, and entitled “PRECISION TIME TRANSFER SYSTEMS AND METHODS IN OPTICAL NETWORKS,” the contents of which are incorporated herein by reference, describes an improvement in accuracy by transferring time in a coherent optics module in a FEC layer to avoid any delay asymmetry introduced therein. Commonly-assigned U.S. patent application Ser. No. 15/878,703, filed Jan. 24, 2018, and entitled “SYSTEMS AND METHODS FOR PRECISE TIME SYNCHRONIZATION WITH OPTICAL MODULES,” the contents of which are incorporated herein by reference, describes an IEEE-1588 transparent clock and/or timestamping inside a digital coherent optical module (e.g., CFP2-DCO) or onboard optics (e.g., COBO). These disclosures address some problems associated with delay asymmetry, using hardware implementations.